The present invention relates to improved methods of completing wells in poorly consolidated and unconsolidated subterranean zones. More particularly, the present invention provides methods for reducing or precluding the migration of fines and sand with the fluids produced from such wells without obstructing the borehole.
Oil and gas wells are often completed in unconsolidated formations containing loose and incompetent fines and sand. This particulate matter commonly migrates with fluids produced by the wells into the production tubing and equipment. The presence of particulate matter in the produced fluids is disadvantageous and undesirable in that the particles abrade pumping and other producing equipment. Additionally, accumulation of particulate matter in gaps between the casing and the producing formation reduces the well's fluid production capabilities.
A common method for improving fluid production from an unconsolidated zone entails fracturing the production zone and holding the resulting fracture open with proppant. Proppant flowback during subsequent fluid production is a common problem. As protection against production of particulate matter entrained within the produced fluids, filters known as gravel packs are commonly used in the production zone of unconsolidated subterranean zones.
In a typical gravel pack completion, a screen is placed in that portion of the wellbore passing through the unconsolidated subterranean zone to be completed. The screen is typically connected to a tool, which includes a production packer and a cross-over. The tool is in turn connected to a work or production string. The gravel pack is formed by pumping graded sand, known in the art as gravel, down the work or production string and through the cross-over as a slurry. The slurry eventually flows into the annulus between the screen and the wellbore wall or casing. The liquid portion of the slurry leaks off into the subterranean zone and/or through the screen, which is sized to prevent passage of the sand in the slurry. As a result, the sand is deposited in the annulus around the screen forming a gravel pack. The size of the sand in the gravel pack is selected such that it prevents formation fines and sand, referred to herein as particulate matter, from flowing into the wellbore with produced fluids.
One common problem encountered during the gravel packing process is the formation of sand bridges. In particular, gravel packing of long and/or deviated unconsolidated producing intervals frequently results in the formation of sand bridges in the annulus. Sand bridges are characterized by non-uniform sand packing of the annulus between the screen and the wellbore. This often occurs as a result of the loss of carrier liquid from the slurry into high permeability portions of the subterranean zone. The loss of fluid causes the formation of sand bridges in the annulus before all the sand has been placed. The sand bridges block further flow of the slurry through the annulus creating voids below the bridges formed. When the well is placed on production, the flow of produced fluids is concentrated through the voids in the gravel pack leading to erosion of the screen and eventually migration of fines and sand with the produced fluids.
In an attempt to prevent the formation of sand bridges in gravel pack completions, special screens with internal bypass tubes have been developed. Such screens have achieved varying degrees of success in avoiding sand bridges; however, these devices and the gravel packing procedure are very costly. Additionally, gravel packs and associated equipment obstruct the borehole. As result, the gravel pack must be removed and replaced anytime lower regions of the well require servicing or accessing for additional completion. Removal and replacement of the gravel packs adds significantly to the costs of well operations.
Thus, there are needs for improved methods of completing wells in unconsolidated subterranean zones whereby the migration of particulate matter with produced fluids can be economically and permanently prevented while allowing the efficient and economical production of hydrocarbons from the unconsolidated producing zone.